1. Field of the Invention
This invention relates to chemically sensitive transducers, and more particularly to a junction field effect transistor (JFET) type of chemically sensitive transducer adapted for detection and measurement of various chemical properties such as ion activity.
2. Prior Art
Field effect transistors (FETs) have recently been proposed for use as the basic or implementing device in chemically sensitive transducers. See, e.g., U.S. Pat. No. 4,020,830 and references cited therein. See also Zemel, Ion-Sensitive Field Effect Transistors And Related Devices, 47 Analytical Chemistry, No. 2, 255 A-68A (1975). The FETs employed in these devices have typically been metal oxide semi-conductor FETs (MOSFET), sometimes refered to as insulated gate FETs, or IGFETs. Because these devices are sensitive to chemical properties of solutions to which they are exposed, they are referred to generally as CHEMFETs.
Prior art CHEMFET transducers (MOSFETs or IGFETs) have relied on a chemically sensitive membrane deposited over the insulated gate. The function of this chemically sensitive membrane is to interact with the solution under test, and to create an electric potential that is proportional to this interaction. This electric potential is in turn sensed at the semiconductor surface of the device, thereby inducing a channel through which current can flow from the source of the FET device to the drain (or vice versa). The electron concentration in the channel that is thus induced, and hence the amount of current that can flow, is a function of the strength of the electric potential, which in turn is a function of the chemical activity occurring at the solution-membrane interface. Current flow between the source and drain of the FET is enhanced by the presence of an electric field at the gate. By selectively choosing the type of membrane that is placed over the insulated gate, unique chemical properties of solutions can be detected by these prior art devices. For example, the presence, concentration, or activity of a particular ion, enzyme, antigen, etc., can be detected by selectively using a particular membrane that would absorb or interact with that particular item. Moreover, use of several different chemically sensitive membranes, each over a different IGFET on the same substrate, thus results in a multi-transducer CHEMFET which could simultaneously sense several selected chemical properties of the solution.